Autoimmune diseases are common and arise when immunological tolerance fails. Cells of the adaptive immune system express a diverse repertoire of antigen receptors, enabling the immune system to respond to a wide range of potential pathogens. However, T and B cells whose antigen receptors recognize self antigens also exist within this repertoire. Multiple modes of immunological tolerance act to eliminate, modify, or restrain such potentially autoreactive lymphocytes. Allelic exclusion is a fundamental mechanism of immunological tolerance, acting to ensure that most mature T cells express only a single antigen receptor specificity. Allelic exclusion is imperfect, however, and T cells that express two productively rearranged TCR? or TCR? chains can be found in both mice and humans. Dual TCR T cells pose two main hypothetical risks to immunological tolerance: such cells might be less susceptible to clonal deletion or they could be activated by recognition of foreign peptide:MHC complexes through one TCR, then provoke autoimmunity through the other (autoreactive) TCR. Because TCR? allelic exclusion is so stringent, prior studies of the role of dual TCR expression in autoimmunity have focused on dual TCR? expression and have concluded that dual TCR expression is not necessary for disease development in several mouse models of autoimmunity. A possible contribution of dual TCR? expression has not been explored. Our preliminary studies demonstrated that incomplete TCR? allelic exclusion can lead to autoimmunity. Expression of dual TCR? (or dual TCR?) chains allowed autoreactive TCR transgenic T cells to escape clonal deletion, culminating in spontaneous autoimmune arthritis. This unexpected finding re-opens the possibility that dual TCR expression can allow autoreactive T cells to escape clonal deletion and provoke autoimmunity. The goal of the proposed project is to determine whether dual TCR expression contributes to autoimmune disease pathogenesis in widely-used mouse models. Importantly, each of these models involves mice with diverse T cell repertoires rather than transgene-encoded TCRs. We propose to use TCR?/TCR? hemizygous mice which are unable to generate dual TCR T cells to determine whether dual TCR T cells are involved in the pathogenesis of experimental autoimmune encephalomyelitis and type I diabetes. These studies are expected to address in a definitive fashion whether incomplete allelic exclusion and the resulting dual TCR expression can contribute to the development of autoimmune diseases. In addition, the studies are expected to form the basis for future work exploring a role for dual TCRs in human autoimmune diseases.